1. Field
This invention relates to phased array antenna systems which produce non-circularly symmetric beams, and more particularly, to improvements in the distribution system of such systems which enables them to electronically stabilize beam orientation and beam shape.
2. Prior Art
Phased array antennas are often configured to produce non-circularly-symmetric beams, such as the fan beam which is much wider in one direction through the beam than in the orthogonal direction through the beam. An example is the typical beam produced by a search radar, which is usually narrow in azimuth and wide in elevation. The search radar's vertical fan-shaped beam is scanned in azimuth to sweep through the full search coverage-volume of space. Most prior art fan beams of this type have been produced by antennas which use specially contoured reflectors that are rotated to steer the beam. The speed of such systems is severely limited by mechanical inertia; however, the inertia may be eliminated through the use of electronically scanned phased array antennas. Unfortunately, prior art phased array fan beams present other problems, most notable of which is distortion, referred to as coning, which occurs as the fan beam is scanned off bore sight.
The cause of coning will be explained with the aid of FIG. 1, which is a three dimensional graphical diagram of the antenna patterns produced by a planar phased array. The diagram of FIG. 1 comprises an X axis 101, a Y axis 102, a Z axis 103, a phased array antenna 104 lying in the X-Y plane, antenna elements 105, representative left end antenna element 106, representative right end antenna element 107, antenna beam direction vector 108 ending at point 120, ray 109 between element 106 and point 120, ray 110 between element 107 and point 120, ray 111 between element 106 and a point 121, ray 112 between element 107 and point 121, ray 113 between element 106 and a point 122, ray 114 between element 107 and the point 122, ray 115 between element 106 and a point 123 and ray 116 between element 107 and the point 123.
The direction vector 108, which emanates from the origin and extends to the point 120, represents the direction of the beam produced by a particular phasing of the elements of the phased array antenna 104. The position of the direction vector may be defined conveniently in two ways. The first way is by means of the direction angles .alpha. 127, .beta. 129 and .theta. 126 which are the angles the direction vector makes with the X, Y, and Z axes, respectively. The second way is through the use of spherical coordinate angles which are the angle .phi. 128 between the projection of the direction vector 118 on the X-Y plane and the X-axis and the angle .theta. between the direction vector and the Z axis.
When the phase of the signals to the two representative antenna elements 106 and 107 is adjusted to direct the beam at point 120, it may be represented by the direction vector 108. Only these two antenna elements will be used for illustrative purposes, it being understood that the remaining elements are similarly phased properly to produce the desired fan beam, the complete array normally comprising rows and columns of elements of which element 106 and 107 are only two. For the phasing required to produce direction vector 108, the beam will appear anywhere the rays from the antenna elements 106 and 107 remain the same in length, as for example, at point 121 where rays 111 and 112 drawn from elements 106 and 107 to point 121 are the same in length as rays 109 and 110 respectively drawn from the same elements to point 120. The locus of points that meets this criteria is the curve 119, which includes the points 120 and 121. This curve represents the coning distortion occurring in conventional fan beam arrays as the beam is scanned off bore sight.
Bore sight is the Z axis in FIG. 1, as it is orthogonal to the X-Y plane in which the antenna array 104 lies. If, for example, it is assumed that the phasing of the signals is changed to produce a direction vector 124, which lies along the Z axis and terminates in point 122, then equal length rays 113 and 114 may be drawn to point 124 from antenna elements 106 and 107. Similarly, if a direction vector 125 is drawn along the X axis to a point 123, equal length rays 115 and 116 may be drawn from antenna elements 106 and 107 to the point 123. The phasing in this case produces a locus of points which lie in the X-Z plane.
The antenna pattern produced by the phasing for direction vector 124, is an undistorted fan beam generally lying in the X-Z plane. The usually narrow, flat pattern shape of this beam can be seen in a cross section through the beam taken in a plane orthogonal to the direction vector 124. On the other hand, a fan beam produced off bore sight, say along the direction vector 108, is distorted because it is bowed lying along the curve 119. The curved pattern of this beam can be seen by taking cross section through the beam in a plane orthogonal to the direction vector 108.
FIG. 2 is a spherical graph showing an undistorted fan beam on bore sight as well as a distorted beam off bore sight. This Figure comprises a spherical graph 201, a zero degrees latitude line 202, a zero degrees longitude line 203 and the intersection 209 of these two lines. The angles of latitude and longitude line are aportioned along these lines with the intersection being taken as the zero reference.
FIG. 2 may be related to FIG. 1 by considering the sphere as being centered about the origin of FIG. 1 and oriented so that the Z axis of FIG. 1 passes through the intersection 209. A plot of the fan beam cross sectional pattern about the intersection 209 with the sphere so oriented provides the bore sight pattern. Typical contour lines 204 and 205 in FIG. 2 are the 3 dB and 10 dB fan beam cross sectional pattern on bore sight with the sphere so oriented. The 3 dB and 10 dB patterns for 45 degrees off bore sight are given by contour lines 206 and 207 respectively. The 10 dB pattern for 90 degrees off bore sight is given by contour line 208. The increase in the distortion due to coning at angles off bore sight is evident from the change from the relatively straight and undistorted pattern 204 on bore sight to the curved and significantly distorted pattern 206 off bore sight.
The distortion of the fan beam as it is scanned could cause operational problems for some applications. In search radars, it could cause the azimuth angle reported for the target to be in error, the magnitude of error depending on the elevation of the target. In a Microwave Landing System (MLS) application, the curvature of the fan beam would cause an erroneous indication of aircraft position.
The principal method of correcting this problem, in the prior art has been by computation. An example is the computed compensation applied to the airborne receivers of microwave landing systems which utilizes the coning type electronically scanned antennas. Since both azimuth and elevation angles are measured in this system, each can be used to compute a correction of the other. Such an approach is used in the Time Reference Scanning Beam (TRSB) system adopted by the International Civil Aviation Organization (ICAO) as the international standard Microwave Landing System (MLS). The obvious disadvantage of this approach is the need for computational capability at each receiver and the penalty in cost, size and weight which it carries. However, there is an additional disadvantage which is more subtle. For very wide angles of scan, the coning is so extreme that there is a loss of coverage; the receivers may totally lose contact with a transmission sent over a widely scanned beam.
A second method of dealing with the coning beam problem in the prior art has been to avoid it entirely by using cylindrical arrays. These naturally produce planar (non-coning) beams as they are scanned. The disadvantage of this approach is that cylindrical arrays require more components than planar arrays and thus are costlier, larger and heavier.
A different type of problem is encountered in many applications where a fan-beam producing antenna is mounted on a vehicle subject to attitude rotations such as pitch and roll. These rotations skew the orientation of the fan beam and could result in loss of intended coverage or in direction-finding errors. Considering again the example of a search radar, pitch or roll of the vehicle on which the radar is mounted could result in a significant difference between the azimuth to a target in stable coordinates (referenced to the vehicle's heading) and azimuth to the target in the antenna's coordinates ("deck-plane" coordinates, also referenced to vehicle heading). This difference will be a function of the target elevation and its relative bearing; the difference, .DELTA., is given by the equation: ##EQU1## where R=roll angle, P=pitch angle, .theta. and .phi. are spherical coordinates of target in a stable reference system which were defined in connection with FIG. 1. If the application is such that the target azimuth and elevation are not independently known, but are to be deduced from the radar's measurement, then this difference between the azimuths in the two coordinate systems is directly a direction-finding error. If a pitch of 10 degrees and then a roll of 24 degrees is assumed, the azimuth error can be as large as 27 degrees if the target is at a 40 degree elevation and at a relative bearing of 60 degrees.
Rotation of a scannable antenna about an axis that is perpendicular to the plane of scan can be compensated by adjustment of the amount of scan, although this also produces a change in shape of coning type beams. For antennas which can only scan in one plane, rotation of the antenna about an axis which is in the plane of scan is not as easily compensated.
The principal prior art method of compensating attitude rotation of the vehicle on which either mechanically or electrically scanned antennas have been mounted has been to mount the antenna on an intermediate platform which is mechanically stabilized. For example, the array antennas used for the U.S. Navy's AN/SLQ-32 shipboard system are mounted to roll stabilized platforms. Such platforms significantly increase the cost, size and weight of the system. A further disadvantage is the decreased system reliability imposed by the added moving parts.
Another prior art method for compensating such rotations of the vehicle is electronic stabilization by appropriately adjusting phase-shifters of an array. Such a method has been described by M. J. Kiss in the paper, "Roll Stabilization of Fan Beams on Airborne Electronically Scanned Phased Arrays" which was presented at the 23rd Annual USAF Antenna Symposium, Oct. 10-11, 1973. In this method, every radiator of a planar array is fed via a phase-shifter. A multi-processor computes those commands for each phase shifter which most nearly stabilize the plane in which the beam is shaped. It does this by computing the equation of the equiphase surface about the antenna which if produced by the antenna would yield the best approximation to the stabilized fan beam. Then it computes the relative phase of radiator excitations (phase-shifter commands) which would produce the best approximations to the desired equiphase surface. In general, the command for each phase shifter is different from that used to drive the others. These commands must be recomputed each time either the beam direction is to be changed or for each significant change in antenna attitude. The disadvantage of this approach is that it requires a phase shifter for every radiator and complex computations to set those phase shifters. Also, this method cannot correct the change in curvature of the fan beam as the beam is effectively steered in a direction orthogonal to the plane of beam shaping to carry out the stabilization process. This is because the Kiss method relies on only the adjustment of the phase of the signal applied to each radiator, whereas beam stabilization, without beam shape changes, requires adjustment of both the amplitude and the phase distribution across the antenna aperture.